This Test Guideline (TG) describes a method to determine the hydrophobicity index (Hy) of nanomaterials (NMs), through an affinity measurement. Hydrophobicity is defined as "the association of non-polar groups or molecules in an aqueous environment which arises from the tendency of water to exclude non-polar molecules". By measuring their binding rate to different engineered surfaces (collectors), Hy expresses the tendency of the NMs to favour the binding to a non-polar (hydrophobic) surface because of its low affinity for water. The method applies to NMs dispersed in an aqueous solution or to NM powders after their dispersions in aqueous solutions, with or without a surfactant, using a recommended protocol.